Use `label` attribute for column descriptions in `get_fx_groups()`

[Aariq]

Just got some ideas to improve this from posit::conf(). Add to source data a "description" column with the short text descriptions from Table 6 in Pankow & Asher. Then, use those descriptions to populate the "label" attributes of the columns returned by get_fx_groups() so the descriptions show up in View() and other places.

Originally posted by @Aariq in #114 (comment)

[Aariq]

I think this is mostly superseded by #114 (although it does not actually implement this label feature)